Transcript Photoresists - The University of Oklahoma Department of

ACTFEL
Alternating Current Thin Film
Electroluminescent Lamps
Objective
 Use photolithography to pattern ITO
 Develop photoresist and etch ITO
 Apply layers of phosphors, dielectric, and silver
 Power up lamp
 Troubleshoot
Preparation and Priming
 Prepare the substrate (glass, with ITO):
Wash with appropriate solvent to remove any dirt
and other impurities
– Acetone, MeOH
Dry with Nitrogen
 Deposit Primer
Chemical that coats the substrate and allows for
better adhesion of the resist, HMDS
Spin-Coating the Resist
 Spin-coat the photoresist onto the surface of the
glass
RPM: 4500
Time: 42 sec
Produces a thin uniform layer of photoresist on the
glass surface.
 Use red/amber safe light at this stage
Soft-Baking

Put on hotplate
Temperature: 115°C, Time: 5 min

Removes volatile solvents from the coating

Makes photoresist imageable

Hardens to amorphous solid

Be careful not to overbake and destroy the sensitizer

Be careful not to not soft-bake
Mask Alignment and Exposure

Photomask is a square piece of transparency printed on by a laser printer

Maximize light intensity using photo-detector

Remove all shadows from exposure area

After alignment, the photoresist is exposed to the UV lamp
Photoresist Developer
Highly pure buffered alkaline solution

Removes proper layer of photoresist upon contact or immersion

Degree of exposure affects the resolution curves of the resist

Immerse wafer in developer and swirl around for 90 seconds

Wash developed wafer with deionized water and dry with
nitrogen
Etching the ITO
 Put glass in a beaker of 20% HCl, 5% HNO3
 Heat beaker for 10 minutes in water bubble bath
 Check for infinite resistance across The Gap
 Fix Gap with razor blade if necessary
 Remove wafers and wash in acetone
 Dry with Nitrogen
Applying Thin Films
After the ITO is patterned the ACTFEL lamp can be made.
 Each layer comes packaged separately as a thick paste (stir before using).
 The thickness of each layer is controlled by using scotch tape as a spacer.
 Apply scotch tape along 3-5mm on two parallel sides of the plate.

 Apply the pastes
in sequence using a spatula.
Thin them by scraping a microscope slide
across the layer.
 Dry and cure each layer before application of
the next
Each layer is dried in an oven at 130°C for 15
minutes.
1st phosphor
2nd dielectric
3rd conductive silver rear electrode
Cross-section of TFEL display
Applying Thin Films 2
The thin films must be applied to the substrate within
defined boundaries to avoid shorting the capacitor.
 Layer Constraints
The phosphor layer should be as thin as possible
The dielectric layer should cover all of the phosphor layer and be as thin as possible
without risking a short in the capacitor.
The silver layer must not touch the ITO. Parts of the ITO layer are removed in order to
extend the silver layer to the edge of the glass. This makes it easier to connect the lamp to
a power source.

Silver Layer (Rear Electrode)
Phosphor Layer
Dielectric Layer
The black lines mark the etched ITO pattern, and are used to accurately place the scotch tape; they’re later removed with acetone.
Powering Lamps

After the thin films are dry, the lamp needs a power source.

Copper tape is used to make good contacts without damaging the lamp.
Small pieces of tape are attached to the ITO layer and the silver layer
separately.

The phosphor requires a changing electric field in order to fluoresce.
Front and back of device
A DC voltage will only produce a changing electric field in a capacitor
as it charges.
In order to produce continuous lighting an AC voltage is required.

Normal 110V 60Hz AC power can be used to light your lamp. In the lab we
use a high frequency power supply 60-2000 Hz and a few hundred volts,
which gives a brighter light.
Device with
leads on,
powered, and
in darkness.